Ni-Cr-Mo alloy having improved corrosion resistance

ABSTRACT

An alloy having a combination of good corrosion resistance in sulfuric acid and resistance to localized attack in chloride-bearing, gas scrubbing systems has a composition in weight percent of 22.0% to 24.0% chromium, 15.0% to 16.5% molybdenum, up to 1.5% iron, 0.14% to 0.46% copper, 0.1% to 0.4% aluminum, up to 0.010% carbon, up to 0.3% cobalt, up to 0.5% manganese, up to 0.10% silicon, up to 0.015% phosphorus, up to 0.010% sulfur, and the balance nickel plus impurities.

FIELD OF THE INVENTION

The invention relates generally nickel base alloys containing significant amounts of chromium and molybdenum along with minor, but important, amounts of other alloying elements which impart general corrosion resistance to the alloys.

BACKGROUND OF THE INVENTION

Ni—Cr—Mo alloys are widely used within the chemical process, pharmaceutical, and flue gas desulfurization industries. Their attributes include high resistance to corrosion in aggressive acids, such as sulfuric, hydrochloric, and hydrofluoric, and, in the case of wrought products (sheets, plates, bars, and tubes, for example), ease of forming and welding. The first commercial Ni—Cr—Mo material was a casting material developed in the early 1930's, and is disclosed in U.S. Pat. No. 1,836,317. This alloy has been sold under the trademark HASTELLOY C and is still used today (albeit with different designations) for intricate castings. Wrought products of HASTELLOY C alloy became widely available in the 1950's.

In the mid-1960's, advances in both melting technology (namely the development of argon-oxygen decarburization techniques) and corrosion science (knowledge of the influence of minor element additions) led to the development of C-276 alloy, a low-carbon, low-silicon wrought version of HASTELLOY C alloy. The main attribute of C-276 alloy (disclosed in U.S. Pat. No. 3,203,792) was its lack of need for a post-weld heat treatment, prior to being placed in service in corrosive environments. In essence, C-276 alloy was less prone than its predecessor to the precipitation of deleterious second phases in heat affected zones (HAZ), during welding.

Subsequent developments of the Ni—Cr—Mo system have been focused on further improvements in thermal stability (i.e. avoidance of second phase precipitates) and/or enhanced resistance to corrosion. Ni—Cr—Mo materials developed since 1970 include HASTELLOY C-4 alloy (U.S. Pat. No. 4,080,201), HASTELLOY C-22 alloy (U.S. Pat. No. 4,533,414), NICROFER 5923 hMo (U.S. Pat. No. 4,906,437), INCONEL 686 alloy (U.S. Pat. No. 5,019,184), and HASTELLOY C-2000 alloy (U.S. Pat. No. 6,280,540). HASTELLOY C-2000 alloy is unique in requiring a copper addition. Copper is added in an amount to provide 1 to 3.5 wt. % copper in the alloy.

According to the manufacturer's material data sheet, NICROFER 5923 hMo alloy, also known as Alloy 59, contains from 22.0% to 24.0% chromium, 15.0% to 16.5% molybdenum, up to 1.5% iron, 0.010% carbon, up to 0.5% manganese, up to 0.10% silicon, up to 0.3% cobalt, 0.1% to 0.4% aluminum, up to 0.015% phosphorus, up to 0.005% sulfur and the balance nickel plus impurities. Copper is not mentioned in the patent covering Alloy 59 (U.S. Pat. No. 4,906,437). Although the UNS compositional specification that applies to Alloy 59 allows a maximum copper content of 0.5 wt. %, the commercial embodiments of Alloy 59 have typically contained about 0.01% copper. Thus, to the extent that the art has considered the influence of copper upon Alloy 59, the art has regarded copper to be either unnecessary or detrimental.

The chromium and molybdenum contents of HASTELLOY C-2000 alloy are similar to the chromium and molybdenum contents of Alloy 59. U.S. Pat. No. 6,280,540, which relates to HASTELLOY C-2000, teaches that copper, within a narrow critical range, can be added to many existing high chromium Ni—Cr—Mo alloys to enhance their resistance to non-oxidizing media. The broadest claimed range for copper in this patent is 1.0 to 3.5%. Since the chromium and molybdenum content of Alloy 59 are similar to that of C-2000 alloy, one would expect that copper should be added to a level of 1.0 to 3.5% to obtain improved corrosion resistance. Surprisingly, when I investigated the influence of copper, I discovered that such an addition is not universally better in terms of corrosion resistance. This is particularly true in environments associated with flue gas desulfurization. To achieve better corrosion properties in an alloy containing 22% to 24% chromium, 15.0% to 16.5% molybdenum, a lesser amount of copper must be added.

SUMMARY OF THE INVENTION

I provide an alloy having a combination of good corrosion resistance in sulfuric acid and resistance to localized attack in chloride-bearing, gas scrubbing systems having a composition in weight percent of 22.0% to 24.0% chromium, 15.0% to 16.5% molybdenum, up to 1.5% iron, 0.14% to 0.46% copper, 0.1% to 0.4% aluminum, up to 0.010% carbon, up to 0.3% cobalt, up to 0.5% manganese, up to 0.10% silicon, up to 0.015% phosphorus, up to 0.010% sulfur and the balance nickel plus impurities.

Such an alloy can be created by modifying commercially available Alloy 59 by adding small amounts of copper to the alloy so that copper is present in the modified alloy at 0.14 to 0.46 weight percent.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph of corrosion rates in sulfuric acid of the tested alloy in which the copper content ranged from less than 0.01 weight percent to 1.63 weight percent.

FIG. 2 is a graph of corrosion rates in Green Death versus copper content in the tested alloy.

FIG. 3 is a graph of normalized corrosion rates in sulfuric acid and Green Death versus copper content in the tested alloy.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To assess the effects of copper upon the corrosion characteristics of Alloy 59, eight experimental alloys were made. The compositional range of Alloy 59 is defined (in wt. %) in the UNS tables in ASTM Publication DS-56H as 22.0% to 24.0% chromium, 15.0% to 16.5% molybdenum, 0.1% to 0.4% aluminum, up to 1.5% iron, up to 0.010% carbon, up to 0.3% cobalt, up to 0.50% copper, up to 0.5% manganese, up to 0.10% silicon, up to 0.015% phosphorus, up to 0.010% sulfur and the balance nickel plus impurities. However, commercially available Alloy 59 contains from 22.0% to 24.0% chromium, 15.0% to 16.5% molybdenum, up to 1.5% iron, up to 0.010% carbon, up to 0.5% manganese, up to 0.3% cobalt, 0.1% to 0.4% aluminum, up to 0.015% phosphorus, up to 0.005% sulfur and the balance nickel plus impurities.

The tested alloys were melted to have a content of the specified elements that was within the commercial embodiment of Alloy 59 for all elements except copper. The aim copper levels were 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 wt. %, the last two falling within the realm of the patent for HASTELLOY C-2000 alloy (U.S. Pat. No. 6,280,540). The aim chromium and molybdenum levels were 23 wt. % and 15.75 wt. % respectively, and the only other elements added deliberately were aluminum (0.3 wt. %), iron (0.5 wt. %), and manganese (0.15 wt. %). This aluminum level is typical of the wrought Ni—Cr—Mo alloys and is used for deoxidation purposes. 0.5 wt. % is a typical iron level for low-iron Ni—Cr—Mo alloys. Manganese is typically added to the nickel alloys at approximately 0.15 wt. % to help with desulfurization.

These experimental alloys were melted in a vacuum induction melting furnace (using a 50 lb. charge weight), and poured into cylindrical molds, to form 2.75 inch diameter electrodes. These were electro-slag remelted, as is normal for wrought Ni—Cr—Mo alloys, into 4 inch diameter ingots. These ingots were soaked for 60 hours at 2200° F. (1204° C.) prior to hot forging, then hot rolled into sheets of thickness 0.125 inch at 2200° F. (1204° C.). The sheets were annealed at 2050° F. (1121° C.), which is the recommended annealing temperature for Alloy 59, and water quenched, prior to corrosion testing. All of the alloys responded well to this annealing treatment; they exhibited microstructures with little or no grain boundary precipitation (of second phases).

The actual compositions of these experimental alloys, based on chemical analyses of samples taken after primary melting, are given in Table 1. From the table, it is evident that the copper content had values of <0.01, 0.14, 0.24, 0.46, 0.7, 1.09, 1.39, and 1.63 wt. %. The chromium content of these experimental alloys varied between 22.48 and 23.12 wt. %, while the molybdenum content ranged from 15.36 to 15.86 wt. %. TABLE 1 EXPERIMENTAL ALLOY COMPOSITIONS & CORROSION TEST RESULTS EN104 EN904 EN204 EN304 EN404 EN1004 EN1104 EN1204 AC- AC- AC- AC- AC- AC- AC- AC- AIM TUAL AIM TUAL AIM TUAL AIM TUAL AIM TUAL AIM TUAL AIM TUAL AIM TUAL Al 0.3 0.3 0.3 0.31 0.3 0.34 0.3 0.35 0.3 0.34 0.3 0.33 0.3 0.36 0.3 0.36 C LAP 0.009 LAP 0.005 LAP 0.008 LAP 0.011 LAP 0.008 LAP 0.003 LAP 0.005 LAP 0.004 Co — 0.008 — <0.01 — 0.006 — 0.006 — 0.004 — <0.01 — <0.01 — <0.01 Cr 23 22.5 23 22.62 23 22.68 23 22.67 23 22.77 23 22.48 23 23.12 23 22.68 Cu LAP <0.01 0.1 0.14 0.2 0.24 0.4 0.46 0.6 0.7 0.8 1.09 1 1.39 1.2 1.63 Fe 0.5 0.53 0.5 0.63 0.5 0.52 0.5 0.51 0.5 0.51 0.5 0.64 0.5 0.64 0.5 0.62 Mn 0.15 0.17 0.15 0.17 0.15 0.16 0.15 0.16 0.15 0.17 0.15 0.17 0.15 0.18 0.15 0.17 Mo 15.75 15.64 15.75 15.69 15.75 15.49 15.75 15.61 15.75 15.74 15.75 15.76 15.75 15.86 15.75 15.36 N — 0.002 — <0.002 — 0.003 — 0.003 — 0.002 — <0.002 — <0.002 — <0.002 Ni BAL 60.78 BAL 60.48 BAL 60.51 BAL 60.16 BAL 59.59 BAL 59.75 BAL 58.82 BAL 59.4 S — 0.004 — 0.001 — 0.004 — 0.004 — 0.003 — <0.001 — 0.002 — 0.002 Si LAP 0.02 LAP 0.03 LAP 0.005 LAP 0.009 LAP 0.02 LAP 0.03 LAP 0.03 LAP 0.02 W — 0.04 — 0.05 — 0.04 — 0.04 — 0.04 — 0.05 — 0.05 — 0.15 20% 1.5 mpy 1.7 mpy 1.4 mpy 1.6 mpy 1.4 mpy 1.5 mpy 1.3 mpy 1.4 mpy H₂SO₄ 93° C. 40% 16.5 mpy 15.5 mpy 18.3 mpy 16.2 mpy 8.8 mpy 13.1 mpy 11.3 mpy 6.9 mpy H₂SO₄ 93° C. 60% 44.7 mpy 29.4 mpy 28.2 mpy 22.1 mpy 20.6 mpy 18.4 mpy 16.6 mpy 14.2 mpy H₂SO₄ 93° C. 80% 109.2 mpy 91.4 mpy 92.9 mpy 88.6 mpy 86.5 mpy 59.5 mpy 32.7 mpy 35.1 mpy H₂SO₄ 93° C. 6% 3.1 mpy 2.5 mpy 3.1 mpy 3.5 mpy 3.4 mpy 2.4 mpy 2.2 mpy 105 mpy FeCl₃ 145° C. 6% 3.8 mpy 2.6 mpy 3.7 mpy 3.9 mpy 8.4 mpy 3.0 mpy 2.9 mpy 679 mpy FeCl₃ 150° C. Green 9.5 mpy 15.6 mpy 344 mpy 480 mpy 963 mpy 612 mpy 892 mpy 1224 mpy Death 130° C. Green 317 mpy 277 mpy 345 mpy 477 mpy 999 mpy 1066 mpy 898 mpy 1092 mpy Death 140° C.

To assess the corrosion characteristics of the experimental materials, they were tested in four concentrations (20, 40, 60, and 80 wt. %) of reagent grade sulfuric acid and a solution known as Green Death, which comprises 11.5% sulfuric acid+1.2% hydrochloric acid+1% ferric chloride+1% cupric chloride. It is well-known that the primary condensate in flue gas desulfurization systems is sulfuric acid, usually at concentrations in excess of 50 wt. %. The Green Death solution is a well known test medium, used to assess the resistance of nickel alloys to localized attack (pitting) in sulfuric acid-based, chloride-bearing, gas scrubbing systems. The sulfuric acid tests were run at 93° C. (a commonly used test temperature for sulfuric acid) and the Green Death tests were run in pressure autoclaves at 130° C. and 140° C. (in the knowledge that such temperatures are required to differentiate between alloys of this type). Additional pitting tests were performed in acidified 6% ferric chloride, at 145° C. and 150° C., according to the procedures defined in ASTM Standard G 48.

The sulfuric acid and Green Death test results generated during this work are also presented in Table 1, and are shown graphically in FIGS. 1 and 2. The results of the ferric chloride tests are reported only in Table 1. Essentially, it was discovered that increasing copper content improves corrosion resistance in 60% and 80% sulfuric acid, but decreases corrosion resistance in the Green Death solution. In particular, the resistance of such alloys to higher concentrations of sulfuric acid appears to be enhanced significantly by a copper addition of just 0.14 wt. %. Thus, one can improve the corrosion resistance of the commercial embodiments of Alloy 59 by adding copper. But contrary to the teaching of the applicable UNS specification, the data indicates that the addition should not be as much as 0.5%. That is so, because pitting resistance in Green Death deteriorates significantly, as the copper content is raised above 0.46 wt. %. Corrosion resistance in ferric chloride was acceptable in all tested alloys except the one alloy with the highest copper content which was 1.63%.

Based upon the aforementioned results, Ni—Cr—Mo alloys with the compositional range of Alloy 59 reported above, but containing copper in an amount within a narrow range of from 0.14 to 0.46 wt. % provide the best corrosion results. Such alloys possess significantly higher resistance to concentrated sulfuric acid than similar alloys free of copper, and possess moderate resistance to pitting, as measured by performance in the Green Death solution at 140° C. To illustrate this point, a plot is presented in FIG. 3 of the normalized corrosion rates versus copper content (in the range 0 to 1.09 wt. %), in 60% sulfuric acid and 140° C. Green Death. The data used to create this plot are given in Table 2. TABLE 2 Values Used in Normalized Corrosion Rate Plot (FIG. 3) MEDIUM EN104 EN904 EN204 EN304 EN404 EN1004 60% 1 0.66 0.63 0.49 0.46 0.41 SULFURIC ACID 93° C. GREEN 0.30 0.26 0.32 0.45 0.94 1 DEATH 140° C.

The normalized values in the case of 60% sulfuric acid were calculated by considering the maximum corrosion rate (44.7 mpy) as unity. In the case of Green Death at 140° C., the corrosion rate (1,066 mpy) of the alloy containing 1.09 wt. % copper (EN1004) was considered unity. While very low levels of copper which might be present as an impurity result in good corrosion resistance to Green Death, corrosion resistance is quite high in 60% sulfuric acid. At higher levels of copper above 0.5 percent, the corrosion performance in sulfuric acid is acceptable but the corrosion rate in Green Death is high. Consequently, the best corrosion performance in sulfuric acid and resistance to localized attack in chloride-bearing, gas scrubbing systems is achieved when small amounts of copper are added so that copper is present in the alloy at 0.14 t0 0.46 weight percent.

Although I have shown and described certain present preferred embodiments of my improved Ni—Cr—Mo alloy and method of making same, it should be distinctly understood that the invention is not limited thereto, but may be variously embodied within the scope of the following claims. 

1. An alloy with good resistance to concentrated sulfuric acid and chloride-induced pitting consisting in weight percent essentially of 22.0% to 24.0% chromium, 15.0% to 16.5% molybdenum, up to 1.5% iron, 0.14% to 0.46% copper, 0.1% to 0.4% aluminum, up to 0.010% carbon, up to 0.3% cobalt, up to 0.5% manganese, up to 0.10% silicon, up to 0.015% phosphorus, up to 0.010% sulfur, and the balance nickel plus impurities.
 2. A method of improving corrosion resistance in sulfuric acid and chloride induced pitting in an alloy having a composition consisting in weight percent essentially of 22.0% to 24.0% chromium, 15.0% to 16.5% molybdenum, up to 1.5% iron, 0.1% to 0.4% aluminum, up to 0.010% carbon, up to 0.3% cobalt, up to 0.5% manganese, up to 0.10% silicon, up to 0.015% phosphorus, up to 0.010% sulfur, and the balance nickel plus impurities comprising adding copper to the alloy thereby forming a modified alloy, the copper being added in an amount so that 0.14 to 0.46 weight percent copper is present in the modified alloy. 